Composite roll for rolling and process for producing the same

ABSTRACT

A composite roll for hot rolling wherein the outer layer portion of a composite roll comprises a steel comprised of, in terms of % by weight, 1.5 to 2.4% of C, 3 to 6% of V and 10 to 20% of at least one element selected from the group consisting of Cr, Mo and W and optionally at least one element of 0.05 to 0.20% of Al and 0.02 to 0.10% of Ti as an inoculation material, with the balance comprising Fe and unavoidable impurities, the outer layer portion comprising a metallic structure which has a crystal grain diameter of 30 to 150 μm and is surrounded by a eutectic carbide crystallized in the grain boundary or a metallic structure wherein a primary crystallized carbide is further dispersed and crystallized in a matrix structure of the crystal. It is excellent particularly in the surface roughening resistance.

TECHNICAL FIELD

The present invention relates to a composite roll for rolling comprisinga solid phase core material and an outer layer material formed bycasting around the core material, and a process for producing the same.

BACKGROUND ART

A high alloy chromium cast iron having suitable wear resistance andcracking resistance and comprising 2 to 3.2% of C, 12 to 18% of Cr, 2%or less of Ni and 2% or less of Mo or a high alloy grain cast ironcomprising 3 to 3.4% of C, 0.4 to 1.5% of Cr, 2 to 5% of Ni and 0.2 to1.0% of Mo has hitherto been used as a roll for hot rolling (see "TekkoZairyo no Mamo (Wear of Steel Material)", Joint Society on Iron andSteel Basic Research, page 16 (1984)). Further, Japanese UnexaminedPatent Publication (Kokai) No. 58-87249 has proposed a wear-resistantcast iron roll material comprising 2.4 to 3.5% of C, 6.1 to 14% of V andalloying elements, that are Cr, Mo, W and Co, for improving the wearresistance.

With respect to a technique for producing this type of roll, asdisclosed in, for example, Japanese Examined Patent Publication (Kokoku)No. 59-19786 and Japanese Unexamined Patent Publication (Kokai) No.61-60256, a technique is known wherein a cooling die is provided arounda core material preheated by means of a preheating coil, introducing amolten metal into the cooling die while heating the molten metal bymeans of a heating coil to form an outer layer welded to around the corematerial.

Wear resistance and crack resistance are important to propertiesrequired of the rolling roll of the type described above. Smaller wearcontributes to an improvement in the accuracy of sheet thickness of therolled product and, at the same time, reduces the frequency ofreplacement of rolls, which in turn contributes to an improvement in theworking efficiency.

When the crack resistance is insufficient, cracking occurs due to theapplication of thermal or mechanical load during use, which gives riseto a major problem trouble. Therefore, it is strongly desired for therolling roll to have both the wear resistance and the crack resistance.

In this connection, as compared with the conventional high chromium castiron and high alloy grain cast iron, a wear-resistant material proposedin Japanese Unexamined Patent Publication (Kokai) No. 58-87249 has beenconfirmed to have better wear resistance. In this material as well, itis necessary to further improve the crack resistance.

Surface roughening resistance and roll surface roughness are otherproperties required of the rolling roll. Specifically, the smaller thesurface roughness of the roll applied to the rolling, the better theappearance of a product formed by rolling through the use of such aroll. For this reason, an improvement in the surface rougheningresistance through a reduction in the surface roughness of the roll hasbeen strongly desired in the art.

DISCLOSURE OF THE INVENTION

In view of the above-described problems of the prior art, an object ofthe present invention is to provide a process for producing a compositeroll for hot rolling which is excellent in wear resistance and crackresistance, particularly surface roughening resistance.

Another object of the present invention is to provide a process for theformation of a composite roll which comprises cladding the periphery ofa core material with an outer layer material.

The present inventors have made extensive and intensive studies with aview to attaining the above-described object and, as a result, havefound that in order to improve the above-described properties in thecomposite roll, it is necessary to use a steel having particularconstituents and crystal structure and that in the process for theformation of a composite roll, it is important to specify coolingconditions (optionally induction heating conditions ) of a molten metal,which has led to the completion of the present invention.

Specifically, the subject matter of the present invention resides in acomposite roll wherein the outer layer portion of a composite rollcomprises a steel comprised of, in terms of % by weight, 1.5 to 2.4% ofC, 3 to 6% of V and 10 to 22% of at least one element selected from Cr,Mo and W and optionally at least of 0.05 to 0.20% of Al and 0.02 to0.10% of Ti as an inoculation material with the balance consisting of Feand unavoidable impurities, said outer layer portion comprising ametallic structure which has a crystal grain diameter of 30 to 150 μmand is surrounded by an eutectic carbide crystallized in the grainboundary or a metallic structure wherein a carbide as a primary crystalis further dispersed and crystallized in a matrix structure of saidcrystal.

According to another aspect of the present invention, there is provideda process for producing a composite roll, comprising pouring a moltenmetal comprising the above-described steel constituents in between arefractory frame and a core material, conducting induction heating,cooling and solidifying the poured molten metal by means of awater-cooled mold provided at the lower end of said refractory frame atan average solidification rate of 4 to 50 mm/min to form an outer layerportion and gradually pulling out the peripheral portion integrated withthe core material.

In the composite roll produced according to the present invention, theouter layer portion has a structure wherein a hard M₆ C carbide(particularly (Cr, Mo, W)₆ C carbide) is crystallized in the grainboundary of a matrix structure (an austenite structure) of a finecrystal grain or a very dense structure wherein a harder MC carbide(particularly VC carbide) is dispersed within the matrix structure ofthe outer layer portion, and the matrix structure is hardened by theheat treatment (hardening and tempering) of the roll, so that it ispossible to provide a roll having a surface roughening resistance and awear resistance superior to those of the conventional roll for hotrolling.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram showing the relationship between the crystal graindiameter and the surface roughness after use of the roll. FIG. 2 is adiagram showing the relationship between the average solidification rateand the crystal grain diameter. FIG. 3 is a diagram showing a change inthe structure in the roll production process according to the presentinvention. FIG. 4 is a microphotograph showing the structure of the rollaccording to the present invention and an explanatory diagram for themicrophotograph. FIG. 5 is a diagram showing the relationship betweenthe frequency of a heating coil and the state of entrainment of foreignmatter. FIG. 6 is a partially cross-sectional diagonal viewschematically showing an apparatus for practicing the process accordingto the present invention. FIG. 7 is a schematic cross-section showing aprincipal part of the apparatus shown in FIG. 6.

BEST MODE FOR CARRYING OUT THE INVENTION

The best mode for carrying out the invention will now be described indetail.

At the outset, the constituents of the outer layer portion of the rollaccording to the present invention will be described.

The reason why the C content is limited to 1.5 to 2.4% is as follows.When the C content is less than the lower limit value, the degree ofcrystallization of a hard carbide is so small that the wear resistanceis remarkably deteriorated, which makes it impossible to improve thewear resistance. On the other hand, when the C content exceeds the upperlimit value, the amount of fragile carbide increases. This causes thecrack resistance to be spoiled and the toughness to be lowered, so thatthe object of the present invention cannot be attained.

In general, it is considered that when the C content is enhanced, theamount of the hard carbide is increased, which contributes to animprovement in the wear resistance. In the present invention, however,since the material of the present invention contains a large amount ofalloyed elements, the morphology and amount of the carbide vary. Thepresent inventors have found that the optimum C content for providingboth the surface roughening resistance and the wear resistance is in therange of from 1.5 to 2.4%.

In this respect, the C content of a material proposed in JapaneseUnexamined Patent Publication (Kokai) No. 58-87249 is so high that thewear resistance unfavorably lowers.

The V content is selected by taking the balance between the V contentand the C content into consideration because V causes a MC carbide (VCin the present invention) having a hardness much larger than that ofcarbides based on a cementite carbide (FeC) or a chromium carbide (Cr₇C₃) to be crystallized. In particular, in the present invention, the VCcarbide is directly crystallized as a primary crystal carbide frommolten metal and a most important factor for regulating the structure.

Specifically, even though the C content is in the range of from 1.5 to2.4%, if the V content is less than the lower limit value, that is, lessthan 3%, a hard VC-based carbide does not crystallize and unfavorablycomes into a matrix structure.

On the other hand, when the C content is 6% or more, since the VCcarbide crystallizes as a primary crystal, the amount of the carbide isincreased, so that the surface roughening resistance is deteriorated.Further, since the specific gravity of the VC carbide is much smallerthan that of the molten metal, the distribution is biased, so that nohomogeneous material is produced. For this reason, the V content shouldfall within the above-described range.

All the allowed elements, that is, Cr, Mo and W, combine mainly with Cto form an eutectic carbide. In the present invention, the eutecticcarbide is a very hard M₆ C carbide. Therefore, this carbide can imparta combination of the wear resistance with the toughness to the material.In order to regulate the amount of the carbide formed, the content of atleast one element of the above-described three elements should belimited to 10 to 22%.

When the lower limit value is 10% or less, the amount of the hardcarbide is so small that the wear resistance becomes insufficient.

On the other hand, when the upper limit value exceeds 22%, the amount ofthe carbide becomes so large that the surface roughening resistance isspoiled. For this reason, in order for the material to have acombination of the surface roughening resistance with the wearresistance, the content should fall within the above-described range.

Part of Cr and Mo is distributed in the matrix structure to increase thehardenability and, at the same time, to have a function of conductingprecipitation hardening particularly at a high temperature.

When an oxide forming element, such as Al or Ti, is inoculated into themolten metal during the roll casting, an oxide, for example, Al₂ O₃ orTi₂ O₃, is formed in the molten metal. This oxide serves as a nucleus,and a VC carbide crystallizes around the nucleus. Therefore, the oxideforming element is important to the dispersion and crystallization ofthe VC carbide. At least one of Al and Ti should be added in an amountof 0.05 to 0.20% for Al and 0.02 to 0.10% for Ti.

Besides the above-described ingredients, Si and Mn which are elementsuseful in a dissolution technique each may be incorporated in an amountof about 0.3 to 1.5% for the purpose of conducting deoxidation of themolten metal.

Further, impurities such as P and S as well may be contained so far asthe content is one found in the case of the conventional casting, thatis, about 0.03% or less. These do not spoil the effect of the presentinvention.

Ni is detrimental to the surface roughening resistance in this type ofroll, so that the Ni content is desirably 1% or less.

In the present invention, Co contributes to an improvement in thehigh-temperature strength and the high-temperature hardness of thematrix within the metallic structure, and the addition of Co in anamount of 0.1 to 10%, preferably 5 to 10%, contributes to a furtherimprovement in the surface roughening resistance and the wear resistanceof the roll.

A description will now be given of a densely solidified structure whichis particularly excellent in the surface roughening resistance.

In the conventional hot rolling, since the surface of the roll becomes ahigh temperature of 600° to 800° C., the matrix structure is temperedand softened. For this reason, in commonly used cast-iron-based rollscomprising a high chromium cast iron, a high alloy grain cast iron orthe like, the matrix structure is worn preferentially over the stablecarbide even at a high temperature, so that the surface of the rollbecomes uneven, which gives rise to surface roughening. Refinement ofthe matrix structure and crystallization and dispersion of a largeamount of a hard carbide in the crystal grain boundary and within thecrystal grain are important to the prevention of the surface roughening.

The present inventors have confirmed that when the surface roughness, Ra(μm), of the roll after use is in the range of from 1.6 to 0.3 μm, thesurface roughening of the roll can be prevented and, at the same time,the occurrence of slippage between the roll and the rolled product canbe inhibited.

Further, they have found that in order to obtain the surface roughnessfalling within the above-described range, it is necessary for thecrystal grain diameter of the roll structure to be in the range of from30 to 150 μm.

This is shown in FIG. 1. In FIG. 1, the ordinate represents the surfaceroughness, Ra (μm), and the abscissa represents the crystal graindiameter (μm). The range of the surface roughness contemplated in thepresent invention is one defined by the mark in the drawing. Thisdrawing indicates that the crystal grain diameter necessary forattaining this surface roughness is in the range of from 30 to 150 μm.

In order to always maintain such a surface roughness, it is necessary toimpart the wear resistance and toughness to the roll. For this reason,in the present invention, a hard M₆ C carbide is crystallized as aeutectic crystal in the crystal grain boundary. Further, a very hard MCcarbide is crystallized as a primary crystal within a crystal grain ofthe matrix structure for the purpose of forming a denser structure.

The process of the formation of the structure according to the presentinvention will now be described through an embodiment wherein theabove-described oxide forming element is inoculated into the moltenmetal.

FIG. 3 is a diagram showing the process of advance of the solidificationin the cooling of a molten metal. A molten metal (L) (step 1) comprisingingredients according to the present invention is cooled to disperse andcrystallize a MC (VC) carbide as a primary crystal from the moltenmetal. The carbide is easily and surely crystallized with an oxide (Al₂O₃ or the like) formed in the molten metal serving as a nucleus (step2).

When the molten metal is further cooled, an austenite (γ₁) iscrystallized as a primary crystal around the MC carbide as the primarycrystal and grows in a dendrite form (step 3).

Then, the residual molten metal (n) solidifies in a eutectic crystalform at an eutectic crystallization temperature, so that eutectic M₆ C((Cr, Mo, W)₆ C) carbides and a eutectic austenite (γ₂) crystallize(step 4)

As described above, in the present invention, the crystal grain diameteris in the range of from 30 to 150 μm. The crystal grain diameter isintended to mean a crystal grain diameter at the time of solidification,that is, as indicated in the step 4, the maximum diameter of a crystalgrain surrounded by an eutectic M₆ C carbide crystallized in the crystalgrain boundary. Therefore, the structure is such that a hard carbide iscrystallized in the crystal grain boundary or within a fine crystalgrain.

A process for producing the roll according to the present invention willnow be described. The composite roll of the present invention isproduced by using an apparatus shown in FIGS. 6 and 7.

As shown in these drawings, a core material 1 in a rod form comprisingan alloy steel, such as SCM440, is provided in such a manner that it canbe vertically moved. A preheating coil 4, a refractory frame 5, aninduction heating coil 6 and a water-cooled mold 7 are provided on aplatform 3 having an opening through which the core material 1 has beeninserted and passed in such a manner that they are coaxially provided inthat order from the top about the core material 1. The core material 1is supported by means (not shown) so that it can be moved downward at aconstant low speed. In the apparatus, at the outset, the core material 1is heated by means of the preheating coil 4. A molten metal 9 comprisinga high speed steel or the like reservoired in a ladle 8 is introducedthrough a nozzle 8a into an annular space defined by the outer peripheryof the preheated core material 1 and the refractory frame 5. The heatingcoil 6 is provided around the refractory frame 5, and the molten metal 9within the refractory frame 5 is heated by the heating coil 6. The lowerend of the refractory frame 5 is in contact with the water cooling mold7, and the molten metal introduced into between the water-cooled mold 7and the core material 1 is gradually solidified to form an outer layer2.

Heating of the molten metal by means of the induction heating coil 6 andcooling by means of the water-cooled mold 7 are most important to theproduction of a composite roll by using the above-described apparatus.Specifically, the above-described heating is important to welding of theouter periphery of the core material 1 to the outer layer portion 2, andthe above-described cooling is important to the formation of a structurehaving a crystal grain diameter of 30 to 150 μm.

The size of the structure, that is, the crystal grain diameter, isdetermined by the solidification rate. Therefore, when refinement of thestructure through a reduction in the crystal grain diameter is intended,it is necessary to increase the solidification rate. In the centrifugalcasting which is the most common production process in the art, however,there is a limitation on the crystal grain diameter due to the relianceupon the removal of heat of the mold. Further, in this case, the size ofthe roll has an effect on the crystal grain diameter. For this reason,the crystal grain diameter is about 200 μm at the smallest in the caseof a roll for hot roll finishing.

By contrast, in the continuous casting process used in the presentinvention, since it is possible to positively conduct cooling with waterthrough a water-cooled mold, the solidification rate can be increased.Specifically, a crystal grain diameter of 30 to 150 μm corresponding toa roll surface roughness, Ra, of 0.3 to 1.5 μm can be attained bypulling out the core material integrated with the outer layer at a rateof 4 to 50 mm/min.

FIG. 2 is a graph showing the relationship between the crystal graindiameter (μm) (ordinate) and the average solidification rate (mm/min)(abscissa) in Example 2. It suggests that in order to attain a crystalgrain diameter of 30 to 150 μm, it is necessary for the solidificationrated to be 4 to 50 mm/min.

FIG. 4 is a microphotograph of the outer layer portion of a rollproduced by a casting process through the use of the above-describedcasting equipment wherein the molten metal is composed mainly of 2.13%of C, 5.13% of Cr, 6.48% of Mo, 5.31% of V, 4.12% of W and 0.10% of Aland casting is conducted at a solidification rate of 20 mm/min. As isapparent from the microphotograph and an explanatory diagram for themicrophotograph, in the structure according to the present invention,the periphery of the crystal grain having a diameter of 80 μm issurrounded by an eutectic carbide, and a carbide as a primary crystal isscatteringly present within the matrix.

When the solidification rate is increased for the purpose of reducingthe crystal grain diameter, the welding between the outer layer and thecore material is often deteriorated. For this reason, as describedabove, it is necessary to supply heat by means of an induction heatingcoil for the purpose of attaining complete welding. In supplying heatthrough induction heating, the molten metal is unfavorably stirred.Therefore, with an increase of electric power for heating, stirringpower increases, so that foreign matter on the surface of the moltenmetal, such as an oxide film material and slag, remains in thesolidified boundary and consequently in the outer layer aftersolidification, which remarkably spoils the quality of the product. Inorder to prevent the occurrence of this phenomenon, it is necessary toincrease the frequency for the purpose of suppressing the stirringpower. As shown in FIG. 5, the occurrence of a defect caused byremaining foreign matter can be prevented when the frequency is 5 kHz ormore.

The composite roll produced by continuous casting is subjected to aconventional hardening treatment. In this case, austenite crystallizedduring solidification becomes a hard martensite and is tempered to givea tempered martensite.

Thus, since the composite roll according to the present invention ishard and has a dense structure, it can be very advantageously used as aroll for hot rolling.

EXAMPLES Example 1

Work rolls for hot roll finishing comprising chemical ingredientsspecified in the columns for Examples 1, 2 and 3 of the presentinvention in Table 1 were produced by continuous casting through the useof an apparatus shown in FIG. 6. The dissolution was conducted in a highfrequency furnace, and steel forging (SCM440) was used as the corematerial. With respect to heat treatment, casting was followed byannealing, hardening and tempering.

                                      TABLE 1                                     __________________________________________________________________________                                         Manufacturing quality                                                                   crystal                                                                            Service quality                                                       hard-                                                                            grain                                                                              surface                              Chemical Ingredients (%)  kind of                                                                              ness                                                                             diameter                                                                           roughness                 No.  Process                                                                             C  Si Mn Ni Cr Mo V    W  carbide                                                                              (Hs)                                                                             (μm)                                                                            (Ra)                                                                                slipping            __________________________________________________________________________    Ex. of                                                                        Present                                                                       invention                                                                     1    continuous                                                                          2.13                                                                             0.62                                                                             0.51                                                                             0.10                                                                             5.13                                                                             6.48                                                                             5.31                                                                              4.12                                                                              MC + M.sub.6 C                                                                       83.0                                                                             150  1.6   free                     casting                                                                  2    continuous                                                                          2.10                                                                             0.63                                                                             0.52                                                                             0.05                                                                             5.36                                                                             6.02                                                                             4.83                                                                              3.98                                                                              MC + M.sub.6 C                                                                       82.0                                                                             80   0.9   free                     casting                                                                  3    continuous                                                                          2.01                                                                             0.59                                                                             0.48                                                                             0.04                                                                             5.52                                                                             6.51                                                                             5.09                                                                              4.01                                                                              MC + M.sub.6 C                                                                       82.5                                                                             50   0.5   free                     casting                                                                  Comp.                                                                         Ex.                                                                           1    centrifugal                                                                         3.30                                                                             0.73                                                                             0.90                                                                             4.42                                                                             1.62                                                                             0.44                                                                             <0.01                                                                             <0.01                                                                             Fe.sub.3 C                                                                           79.0                                                                             200  2.5   free                     casting                                                                  2    centrifugal                                                                         2.81                                                                             0.58                                                                             1.11                                                                             1.29                                                                             17.44                                                                            1.27                                                                             0.20                                                                              <0.01                                                                             M.sub.7 C.sub.3                                                                      72.0                                                                             180  2.0   free                     casting                                                                  3    forging                                                                             2.09                                                                             0.67                                                                             0.53                                                                             0.15                                                                             5.39                                                                             4.13                                                                             5.02                                                                              0.58                                                                              MC + M.sub.7 C.sub.3                                                                 82.0                                                                             240  3.0   free                4    powder                                                                              2.27                                                                             0.52                                                                             0.70                                                                             0.09                                                                             6.21                                                                             7.70                                                                             4.59                                                                              3.55                                                                              MC     82.5                                                                             25   0.2   occurred                 metallurgy                                                               5    powder                                                                              2.35                                                                             0.53                                                                             0.63                                                                             1.31                                                                             16.02                                                                            1.31                                                                             0.18                                                                              <0.01                                                                             M.sub.7 C.sub.3                                                                      73.0                                                                              5   0.1   occurred                 metallurgy                                                               __________________________________________________________________________

The results of comparison of Examples 1 to 3 of the present inventionwith Comparative Examples 1 to 5 on specific manufacturing quality andservice quality in an actual machine are given in Table 1 . In Examples1, 2 and 3 of the present invention, the crystal grain diameters were150 μm, 80 μm, and 50 μm,, respectively, and the surface roughnesses,Ra, after use were 1.6 μm, 0.9 μm and 0.5 μm respectively. That is,rolls having a very good quality could be produced. Further, no slippagebetween the roll and the rolled product occurred.

Example 2

Work rolls for hot roll finishing wherein the chemical ingredients ofthe outer layer portion and the core material are as specified inExamples 1 to 3 of Table 2 were produced by continuous casting throughthe use of an apparatus shown in FIG. 6. The manufacturing conditions,manufacturing quality and service quality are given in Table 2 togetherwith those of Comparative Examples 1 to 5.

                                      TABLE 2                                     __________________________________________________________________________                   Classifi-                                                                           Chemical Ingredients (wt. %)                             No.     Process                                                                              cation                                                                              C   Si Mn P   S  Ni Cr  Mo V   W    Co                   __________________________________________________________________________    Ex. of Present                                                                Invention                                                                     1       Continuous                                                                           outer 2.05                                                                              0.61                                                                             0.53                                                                             0.014                                                                             0.012                                                                            0.03                                                                             4.85                                                                              6.40                                                                             5.19                                                                              4.86 0.91                         casting                                                                              layer                                                                         core  0.42                                                                              0.25                                                                             0.75                                                                             0.015                                                                             0.007                                                                            0.13                                                                             1.04                                                                              0.17                                                                             --  --   --                                  material                                                       2       Continuous                                                                           outer 2.00                                                                              0.58                                                                             0.51                                                                             0.015                                                                             0.011                                                                            0.04                                                                             4.93                                                                              6.38                                                                             5.08                                                                              4.66 7.81                         casting                                                                              layer                                                                         core  0.41                                                                              0.28                                                                             0.81                                                                             0.015                                                                             0.010                                                                            0.13                                                                             1.02                                                                              0.18                                                                             --  --   --                                  material                                                       3       Continuous                                                                           outer 2.03                                                                              0.57                                                                             0.51                                                                             0.014                                                                             0.013                                                                            0.03                                                                             5.49                                                                              6.52                                                                             5.10                                                                              4.03 3.06                         casting                                                                              layer                                                                         core  0.39                                                                              0.26                                                                             0.77                                                                             0.014                                                                             0.012                                                                            0.14                                                                             1.05                                                                              0.17                                                                             --  --   --                                  material                                                       Comp. Ex.                                                                     1       Centrifugal                                                                          outer 3.28                                                                              0.82                                                                             0.90                                                                             0.015                                                                             0.012                                                                            4.41                                                                             1.68                                                                              0.46                                                                             --  --   --                           casting                                                                              layer                                                                         core  3.25                                                                              1.75                                                                             0.44                                                                             0.040                                                                             0.013                                                                            0.77                                                                             0.34                                                                              0.18                                                                             --  --   Mg 0.052                            material                                                       2       Centrifugal                                                                          outer 2.85                                                                              0.59                                                                             1.09                                                                             0.018                                                                             0.011                                                                            1.26                                                                             17.69                                                                             1.09                                                                             0.20                                                                              --   --                           casting                                                                              layer                                                                         core  3.32                                                                              3.15                                                                             0.45                                                                             0.023                                                                             0.006                                                                            0.50                                                                             0.07                                                                              0.01                                                                             --  --   Mg 0.043                            material                                                       3       continuous                                                                           outer 2.53                                                                              0.60                                                                             0.51                                                                             0.014                                                                             0.011                                                                            0.03                                                                             5.23                                                                              6.12                                                                             6.04                                                                              4.51 --                           casting                                                                              layer                                                                         core  0.40                                                                              0.26                                                                             0.79                                                                             0.014                                                                             0.008                                                                            0.12                                                                             1.05                                                                              0.18                                                                             --  --   --                                  material                                                       4       powder --    2.27                                                                              0.52                                                                             0.70                                                                             0.020                                                                             0.014                                                                            0.09                                                                             6.21                                                                              7.70                                                                             4.59                                                                              3.55 --                           metallurgy                                                            5       powder --    2.35                                                                              0.53                                                                             0.63                                                                             0.018                                                                             0.015                                                                            1.31                                                                             16.02                                                                             1.31                                                                             0.18                                                                              <0.01                                                                              --                           metallurgy                                                            __________________________________________________________________________    Manufacturing                                Service quality                               aver-     Manufacturing quality ton of                                                                             average                                  age frequency           entrain-                                                                              rolling                                                                            crack    slip                            solidifi-                                                                         of coil             ment of based on                                                                           depth    be-                    pour-                                                                             inocu-                                                                             cation                                                                            for heat- dis- crystal                                                                            foreign consump-                                                                           at the                                                                             surface                                                                           tween                  ing lation                                                                             rate                                                                              ing molten                                                                          kind of                                                                           persion                                                                            grain                                                                              matter                                                                             hard-                                                                            tion of                                                                            time of                                                                            rough-                                                                            roll                   temp.                                                                             materi-                                                                            (mm/                                                                              metal car-                                                                              of MC                                                                              diameter                                                                           in   ness                                                                             1 mm accident                                                                           ness                                                                              and                No. (°C.)                                                                      al   min)                                                                              (KHz) bide                                                                              carbide                                                                            (μm)                                                                            product                                                                            (Hs)                                                                             (ratio)                                                                            (ratio)                                                                            (Ra)                                                                              sheet              __________________________________________________________________________    Ex. of                                                                        Present                                                                       Inven-                                                                        tion                                                                          1   1570                                                                              Al    4  5     MC +                                                                              dispersed                                                                          150  free 85.0                                                                             9255 0.6 mm/                                                                            1.5 free                       0.1%           M.sub.6 C                                                                         in matrix         Ton  time                            (Solid)                                                                           --   --  --    --  --   --           (5.30)                                                                             (0.50)                      2   1580                                                                              Al   20  5     MC +                                                                              dispersed                                                                           80  free 84.0                                                                             10182                                                                              0.4 mm/                                                                            0.7 free                       0.1%           M.sub.6 C                                                                         in matrix         Ton  time                            (Solid)                                                                           --   --  --    --  --   --           (5.83)                                                                             (0.33)                      3   1570                                                                              Ti   40  8     MC +                                                                              dispersed                                                                           40  free 85.0                                                                             10532                                                                              0.4 mm/                                                                            0.4 free                       0.05%          M.sub.6 C                                                                         in matrix         Ton  time                            (Solid)                                                                           --   --  --    --  --   --           (6.03)                                                                             (0.33)                      Comp.                                                                         Ex.                                                                           1   1310                                                                              Ca--Si                                                                              3  --    Fe.sub.3 C                                                                        --   230  --   79.5                                                                             1746 1.2 mm/                                                                            2.8 free                       0.15%                                Ton  time                            1340                                                                              --   --  --    --  --   --           (1.00)                                                                             (1.00)                      2   1375                                                                              free  5  --    M.sub.7 C.sub.3                                                                   --   200  --   74.5                                                                             2734 1.8 mm/                                                                            2.5 free                                                            Ton  time                            1350                                                                              --   --  --    --  --   --           (1.57)                                                                             (1.50)                      3   1550                                                                              free 2.5 5     MC +                                                                              segre-                                                                             180  free 85.0                                                                             6192 1.5 mm/                                                                            2.0 free                       0.1%           M.sub.6 C                                                                         gated             Ton  time                                                   in grain                                                                      boundary                                               (Solid)                                                                           --   --  --    --  --   --           (3.55)                                                                             (1.25)                      4   --  --   --  --    MC  --    25  --   82.5                                                                             --   --   0.2 oc-                                                                           curred             5   --  --   --  --    M.sub.7 C.sub.3                                                                   --    5   --   73.0                                                                             --   --   0.1 oc-                                                                           curred             __________________________________________________________________________

The composite rolls of the present invention were excellent in themanufacturing quality as well as in the service quality.

Specifically, the wear resistance of the composite rolls comprisingchemical ingredients falling within the scope of the present inventionwas 5 times or more that of the conventional rolls. Further, thecomposite rolls of the present invention had a small surface roughnessafter use and an improved surface roughening resistance.

In Examples 1, 2 and 3 of the present invention, the crystal graindiameters were 150 μm, 80 μm and 50 μm, respectively, and the surfaceroughnesses, Ra, after use were 1.6 μm, 0.9 μm and 0.5 μm respectively.The results became better with a reduction in the crystal graindiameter. The composite rolls of Comparative Examples 4 and 5 wereproduced by powder metallurgy. In these composite rolls, since thestructure is excessively fine, the surface roughness after use was sosmall that slippage occurred between the roll and the rolled productduring rolling. As a result, these comparative composite rolls wereunsuitable for use.

INDUSTRIAL APPLICABILITY

The application of the present invention to a roll for hot rolling makesit possible to provide a high-quality roll having a good wear resistanceand free from the occurrence of crack or the like derived from lack ofthe toughness. In particular, with respect to the surface rougheningresistance, it was confirmed that the roll of the present invention ismuch superior in the performance to the conventional rolls, whichrenders the roll of the present invention very useful from the viewpointof industry.

We claim:
 1. A composite roll for rolling, comprising a core materialhaving an outer periphery welded to the inner periphery of an outerlayer portion, characterized in that said outer layer portion comprisesa steel comprised of, in terms of % by weight, 1.5 to 2.4% of C, 3 to 6%of V and 10 to 22% of total sum of three elements of Cr, Mo and W, withthe balance comprising Fe and unavoidable impurities, said outer layerportion comprising a metallic structure which has a crystal graindiameter of 30 to 150 μm and is surrounded by a eutectic carbide of M₆ Ctype crystallized in a boundary of the crystal grain, and which has aprimary crystal carbide of MC type in the grain.
 2. A composite rollaccording to claim 1, wherein at least one of 0.05 to 0.20% of Al and0.02 to 0.10% of Ti is added as an inoculation material to chemicalconstituents of said steel and a carbide as a primary crystal iscrystallized within a crystal grain constituting the structure of saidouter layer portion with an oxide formed in a molten metal serving as anucleus.
 3. A composite roll according to claim 1 or 2, wherein saidouter layer portion further comprises 0.1 to 10% of Co.
 4. A compositeroll according to claim 1 or 2, wherein said outer layer portion furthercomprises 5 to 10% of Co.
 5. A process for producing a composite roll,characterized by comprising pouring a molten metal comprised of, interms of % by weight, 1.5 to 2.4% of C, 3 to 6% of V and 10 to 22% oftotal sum of three elements of Cr, Mo and W, with the balance comprisingFe and unavoidable impurities into between a refractory frame and a corematerial, heating the poured molten metal, cooling and solidifying themolten metal by means of a water-cooled mold provided at the lower endof said refractory frame at an average solidification rate of 4 to 50mm/min to form an outer layer portion and gradually pulling out theperipheral portion integrated with the core material.
 6. A processaccording to claim 5, wherein casting is conducted after at least one of0.05 to 0.20% of Al and 0.02 to 0.10% of Ti is added as an inoculationmaterial to chemical consistuents of said molten metal.
 7. A processaccording to claim 5 or 6, wherein said molten metal further comprises0.1 to 10% of Co.
 8. A process according to claim 5 or 6, wherein ahigh-frequency current having a frequency of 5 kHz or more is suppliedto a heating coil provided on the outer periphery of said refractoryframe to heat the molten metal.